1. Field of the Invention
The present invention relates to a wireless communication method and device, especially to a wireless communication method and device capable of improving network throughput.
2. Description of Related Art
Generally speaking, a wireless communication device transmits a packet through a certain frequency channel among one or more prescribed frequency channels. During its transmission, if another device transmits a packet through the same channel, the packets from the two devices will interfere with each other and one or both of the transmissions would probably fail. This interference is called packet collision.
In order to prevent packet collision, a Carrier Sense Multiple Access (CSMA) protocol such as the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is adopted. Take a contention-based system (e.g. a system conforming to a IEEE 802.11 standard) in compliance with the CSMA protocol for example; once a wireless communication device wins a transmission opportunity under the protocol and starts its transmission, any other device running after the same transmission opportunity has to hold off its transmission procedure for the duration indicated by a signal from the winner device, so that a potential collision could be avoided.
However, if the wireless communication device suspends its transmission procedure according to a signal whose strength is too low to make any interference, such suspension will do nothing good but cause a waste of network throughput. Therefore, a threshold for determining whether the suspension is necessary is used. More specifically, in a wireless network based on said CSMA protocol, a wireless communication device (hereafter, concerned device) which shares the same frequency channel with other devices will listen to signals from the other devices in the channel. Upon receiving a signal, the concerned device will measure the strength of the signal and compare it to a predetermined threshold (i.e. Clear Channel Assessment Threshold, CCA threshold) defined by the protocol. If the strength (hereafter, CCA level) of the signal falls below the CCA threshold, the signal could be treated as noise or something negligible; meanwhile, the concerned device may act as though the channel is clear, and either transmit or carry out other appropriate steps that are permitted to it. On the contrary, if the CCA level of the received signal is above the CCA threshold, this signal should be taken valid; in the meantime, the concerned device is required to treat the channel as it is occupied by a valid signal according to the protocol, and may not transmit for the duration indicated by the received signal.
Through the above-mentioned way, the protocol provides some assurance that valid signals will not collide with each other at levels which would cause unsuccessful transmission. Signals that are detected at a strength level higher than a prescribed threshold should be given precedence, so that these signals can be sent to their destination(s) without interference. On the other hand, signals that are detected at a strength level lower than the prescribed threshold could be ignored because these may be noise or signals from devices that are too far to make interference or get interference; in this case, even though a receiving device discovers signals and still transmits, its transmission may not appreciably interfere with the precedent transmission.
Please note that each device conforming to the CSMA protocol has to decide its own CCA threshold in light of the protocol (in other words, in pre-existing standards the value of a CCA threshold is usually defined by the protocol), and the value chosen for a CCA threshold may greatly affect overall network throughput. For some configuration of devices and the environment where they stay, a high CCA threshold applied to one or more devices may lead to a higher aggregate network data rate if multiple transmissions can therefore be carried out simultaneously and successfully. However, for some other configuration of devices and the environment they stay in, a high CCA threshold applied to one or more devices may cause a worse aggregate network data rate if multiple transmissions can be carried out simultaneously but most or all of them fails due to interference (i.e. packet collision); in this case, a low CCA threshold would be a better choice instead.
In light of the above, high (i.e. loose) CCA thresholds may increase overall system throughput significantly in moderately crowded environments because devices do not need to defer for precedent transmissions that may belong to a neighboring network. On the contrary, low (i.e. strict) CCA thresholds provide the stronger level of protection for individual transmissions, and minimize the probability of packet loss due to co-channel interference. Hence, there is a tradeoff: an overstrict CCA threshold prevents waiting devices from transmitting concurrently even when these concurrent transmissions would be successful and cause no failure to other transmissions, while an over loose CCA threshold allows too many devices to transmit simultaneously, which possibly causes all transmissions to fail.
Please also note that whether a permitted transmission (i.e. transmission in light of said transmission opportunity) is successful is dependent on the intended receiver (i.e. destination receiver); but whether a concurrent transmission could be carried out is determined by the result of an unintended receiver comparing the signal strength of the permitted transmission with its CCA threshold. Since the transmitter of the permitted transmission and the intended receiver are two distinct devices, a confusion may arise when the unintended receiver found that its concurrent transmission is allowed in accordance with the signal strength of the transmitter but had no idea whether this concurrent transmission will be harmful to the intended receiver. For instance, as shown in FIG. 1, a device 110 transmits packets to a device 120 while a device 130 plans to send packets to a device 140. If an attenuating obstacle 150 stands between devices 110 and 130 but outside the signal paths between devices 120 and 130 and devices 140 and 130, device 130 may find the signal strength of the packet from the device 110 low enough (i.e. below the CCA threshold of device 130) due to the attenuating obstacle 150 existence and then start transmitting packets to device 140; in the meantime, device 110 keeps sending packets to device 120, but the reception at device 120 and device 140 will be interfered by the transmission from device 130 and device 110 respectively because of the absence of any attenuating obstacle. In brief, under the current CSMA protocol, what is available to device 130 is the interference level caused by device 110 at device 130. In essence what is important to device 130 is the measure correlated with successful transmission.
Another problem of the current CCA mechanism is that an over loose CCA threshold may allow new transmission to succeed, but cause the already on-air transmission to fail. This problem is potentially more serious, as the offending device (with the over loose CCA threshold) may not suffer its own packet loss, and thus lack an immediate feedback which could help change its behavior. This monopoly-like behavior may cause severe fairness problems because the suffering devices continually lose packets due to interference from the offending device but have no ways to communicate that fact to the offending device.
In summary, how to appropriately adjust the CCA threshold of a wireless network device, to enhance the total throughput of the network it belongs to, is a topic of great importance to the wireless networking industry.